The invention relates to an improved process for preparing a heat stable pigment violet 23.
Pigment violet 23 has been known for decades and in the course of this time has acquired a very great importance because of its outstanding properties as a colorant. It is therefore manufactured on an industrial scale in large quantities. As long as the colorant has been known, the principal features of its manufacture have remained the same. It is typically synthesized by reacting chloranil (tetrachloro-p-benzoquinone) with 3-amino-9-ethylcarbazole to give 2,5-di-(9-ethylcarbazol -3-ylamino)-3,6-dichloro-1, 4-benzoquinone. This is followed by cyclisation to give the pigment violet 23. (See Venkataraman, The Chemistry of Synthetic Dyes, Volume II (1952), pages 786 and 787). The following equations serves to illustrate the synthetic reaction scheme: 
Pigment Violet 23
While many variations in the solvents used and conditions of this reaction have been disclosed, invariably, the reaction also leads to the formation of several by-products that the prior art deemed contaminants. These by-products are normally washed away from the crude pigment violet 23 before any conditioning (i.e. heat stabilizing) of such crude occurs.
The present invention provides an improvement in the process for the conditioning of pigment violet 23 having the formula: 
the improvement comprises carrying out said conditioning in the presence of about 1 to about 10 wt. %, preferably about 5 to 8 wt. %, relative to the weight of the crude pigment, of at least one compound selected from the group consisting of: 
wherein each R is independently selected from either H or CH3.
It has been surprisingly found that the heat stability of pigment violet 23 in general and more specifically in plastics applications is increased significantly when a crude pigment violet 23 product is conditioned in the presence of about 1-10 wt. % of at least one of the following compounds: 
wherein each R is independently selected from either H or CH3.
The conditioning of crude pigment violet 23 may be carried out by various processes such as by grinding the crude pigment in the presence of a grinding agent and an organic solvent. Preferably, the grinding agent is sodium chloride and the organic solvent is diethylene glycol.
Compounds 1, 2 and 3 are formed in final step in commercial preparation of pigment violet 23 crude which involves ring closure of the intermediate di-anil (4) 
using benzenesulfonyl chloride or p-toluenesulfonyl chloride (or other arylsulfonyl chlorides).
It is well known that cyclization of the di-anil intermediate (4) during the synthesis of crude violet 23 results in the concurrent formation of by-product compounds (1), (2) and (3). More specifically, in compounds (1), (2) and (3), R is H when benzenesulfonyl chloride is employed as the cyclizing agent and R is CH3, when p-toluenesulfonyl chloride is employed as the cyclizing agent. Alternatively, a mixture of (1), (2) and (3) can be produced wherein R is a mixture of H and CH3 . This is the case when a mixture of benzenesulfonyl chloride and p-toluenesulfonyl chloride are employed as cyclization agents. When aryl sulfonyl chlorides including other isomers of toluenesulfonyl chlorides, naphthalenesulfonyl chlorides, anthracenesulfonyl chlorides or other aryl systems are employed as the cyclization agents, the corresponding by-products are also formed and result in the same heat stabilizing effect.
The amount of these by-products [i.e. compounds (1), (2), and (3)] retained in the isolated crude product may be controlled by the washout procedure applied during the isolation/filtration process. Thus, rigorous washing with the hot reaction solvent will remove virtually all of the by-product compounds while more by-product compounds are produced under less rigorous washing conditions with preferably 1-10 wt. % retained. Alternatively, the appropriate level of (1), (2) or (3) by-product compounds or mixtures thereof (hereinafter referred to as xe2x80x9cby-product mixturexe2x80x9d), may be added to a purified crude sample prior to its conditioning. The presence of at least one of compounds (1), (2) or (3) in an amount of about 1-10 wt. % causes significant inhibition of recrystallization during the conditioning process. The result is that pigments possessing these levels of by-products (or impurities) also exhibit correspondingly higher surface areas.
For example, when a crude product containing less than 1 wt. % of at least one of compounds (1), (2) or (3) is conditioned, the resulting surface area is typically in the range of 85 to 95 m2/g. By contrast, when a crude product containing from about 1 to 10 wt. %, preferably from about 5 to 8 wt. % of at least one of compounds (1), (2) or (3) is conditioned, the resulting surface area is typically in the range of 115 to 125 m2/g. The superior heat stability of the latter product in plastic, compared to the former is also due to the presence of one or more of these compounds. Thus, crystalline growth (or recrystallization) of the pigment is significantly inhibited, during the high temperature processing necessary for the coloration of plastics.
The examples that follow demonstrate that the heat stability exhibited by plastic colored with the pigment prepared from a crude pigment violet 23 containing from 5 to 10 wt. % of at least one of compounds (1), (2) or (3) is superior to that of the equivalent plastic colored with the pigment prepared from crude pigment containing less than 1 wt. % of the same compound(s).